Raising a child has many rewards, but also a lot of downsides, including that it messes up parents' immune systems. Photo: Getty Images

Researchers at VIB and KU Leuven in Belgium and the Babraham Institute in Britain made the discovery by accident after undertaking detailed examination of the immune systems of 670 people, aged from two to 86.

The aim was to learn what drives variation in immunity between individuals – and what happens to the immune systems of two people in close contact.

Over a three-year period, the subjects were regularly monitored, and their immune systems were placed under stress via exposure to a flu vaccine or gastroenteritis infection.

Overall it was found that immune systems tend to bounce back to their original state after stress, indicating elastic potential in our immunity – although age is a crucial factor in immunological health.

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That is, it supported studies that show an age-related decline in response to vaccination and reduced resistance to infection.

But the study looked at a range of factors, including gender, mental and emotional health and obesity – and one of the most significant factors was when two people were co-parenting a child.

The net effect was that two individuals who lived together and raised a child undergo a rewiring of their immunity – such that there is a 50 per cent reduction in the variation between their immune systems, compared with the diversity seen in the wider population.

According to Dr Adrian Liston, a researcher at VIB and KU Leuven who co-led the research, this was the first study of the immune profiles of two unrelated individuals in a close relationship.

"Since parenting is one of the most severe environmental challenges anyone willingly puts themselves through," Dr Liston has reportedly said.

"It makes sense that it radically rewires the immune system. Still, it was a surprise that having kids was a much more potent immune challenge than severe gastroenteritis. That's at least something for prospective parents to consider – the sleep deprivation, stress, chronic infections and all the other challenges of parenting does more to our body than just give us grey hairs."